1. Field of the Invention
The present invention relates to a liquid crystal display device in which the viewing angle characteristic of a display screen is improved, and particularly to a liquid crystal display device having, as a characteristic feature, a helical pitch between a liquid crystal layer and a TN liquid crystal layer of a color liquid crystal display panel having nematic liquid crystal display elements.
2. Description of Related Art
In a general liquid crystal display device, light from a light source is modulated to perform an image display. A visual angle is offset from the normal direction of the liquid-crystal panel due to the birefringence of a liquid crystal, retardations are different from each other. For this reason, visual angle dependency is large.
Conventionally, it is proposed to widen a visual angle at which a correct image can be seen. For example, as a first prior art, a liquid crystal display device planning an improved visual angle characteristic is disclosed in Japanese Unexamined Patent Publication No.8-95034 (Patent No.2706902) published on Apr. 12, 1996.
In the first prior art, a phase plate having an optically negative uniaxial property is arranged in a color liquid crystal display device. In addition, the optical axis of the phase plate is set to be inclined from the normal direction of a film by 5 to 50xc2x0.
More specifically, in a twisted nematic type (TN type) color liquid crystal display device, twisted nematic type liquid crystal cells, each of which is constituted by a color filter, a pixel electrode, and a nematic liquid crystal substantially having a twisted angle of 90xc2x0, are arranged. Polarizing plates are arranged both the sides of the liquid crystal cell, and the phase plate described above is sandwiched between the liquid crystal cell and one of the polarizing plates. The phase plate has a minimum value of the absolute value of a retardation which is not zero, and the direction indicating the minimum value is inclined from the normal direction by 5 to 50xc2x0.
A plurality of stripe electrodes are arranged to cross each other, and pixels are formed at the crossing portions of the electrodes.
The phase plate is an optical anisotropic element having an optically negative uniaxial property as described above. The optical axis of the phase difference plate is inclined from the normal direction of a film by 5 to 50xc2x0. The light transmittance of the phase difference plate is 80% or more. Assume that principal refractive indexes in a film surface are represented by nx and ny, that a refractive index in a direction of thickness is represented by nz, and that a thickness thereof is represented by d. In this case, the principal refractive indexes of the three axes preferably have preferably the following relationships.
Expression 1 is preferably satisfied, a retardation expressed by Expression 2 is preferably 20 nm to 400 nm, and front retardation expressed by Expression 3 is preferably 50 nm or less.
nz less than ny=nxxe2x80x83xe2x80x83(1)
(((nx+ny)/2)xe2x88x92nz)xc3x97dxe2x80x83xe2x80x83(2)
|nxxe2x88x92ny|xc3x97dxe2x80x83xe2x80x83(3)
It is mentioned that a disk-like compound is preferably contained as an optical anisotropic element. For example, the compound has the following structure. That is, a benzene derivative, a cyclohexane derivative, or an aza crown based or phenylacetylene based macro cycle is used as a mother core, and straight chain alkyl groups, alkoxy groups, substitutional benzoyloxy groups, or such are radially substituted as a straight chain.
In this manner, a liquid crystal display device, having a wide viewing angle, in which degradations of image quality such as degradation of contrast, gradation inversion, and a change in hue caused by a visual angle are reduced, can be obtained.
As a second prior art, a liquid crystal display device having as its object to improve the viewing angle characteristic of a display screen is disclosed in Japanese Unexamined Patent Publication No.9-120005 published on May 6, 1997.
Although the second prior art is similar to the first prior art, the first prior art and the second prior art are different from each other in characteristics of phase plates. In the phase plate of the second prior art, a principal refractive index na exists in a plate-like flat surface. A principal refractive index nc (naxe2x89xa0nc) exists in direction which is orthogonal to the principal refractive index na and clockwise or counterclockwise inclined by an angle xcex8 from the plate surface about the principal refractive index na along a plane vertical to the plate surface. In addition, a principal refractive index nb which is smaller than the principal refractive indexes na and nc exists in a direction which is clockwise or counterclockwise inclined by the angle xcex8 along the plane from the normal direction of the plate surface about the principal refractive index na along the plane.
In the phase plate, when a visual angle is inclined from a screen normal direction to a positive visual angle direction, a coloring phenomenon that a displayed image is colored at an angle larger than a predetermined angle and/or an inversion phenomenon that black and white are inverted occurs. On the other hand, when the visual angle is inclined in a reversed visual angle, contrast is sharply degraded.
In the second prior art, for example, a nematic liquid crystal material having a refractive index anisotropy xcex94n of 0.08 is used for a liquid crystal layer, and the thickness of the liquid crystal layer is set to be 4.5 xcexcm. As the phase plate, a plate having a biaxial property and obtained by extending a macromolecular material such as polycarbonate or polyester is used. A first retardation value which is a product ((ncxe2x88x92na)xc3x97d) of the difference between the principal refractive indexes nc and na, and the thickness d in the phase plate is 20 nm. A second retardation value which is a product ((ncxe2x88x92nb)xc3x97d) of the difference between the principal refractive indexes nc and nb, and the thickness d is 135 nm. In addition, a relationship (naxe2x89xa0nc greater than nb) is satisfied. Furthermore, an inclination direction angle xcex8 is 40xc2x0.
As the third prior art, a liquid crystal display device having an improved visual characteristic is also disclosed in Japanese Unexamined Patent Publication No.6-43452 published on Feb. 18, 1994.
In the third prior art, the twisted angle of a twisted nematic liquid crystal is set to be smaller than 80xc2x0. The refractive index anisotropy xcex94n of the liquid crystal layer is set to be 0.02 to 0.12. In addition, the retardation xcex94nxc2x7d (xcex94nxc3x97d) of the liquid crystal layer is set to be 200 to 1,200 nm, and the retardation xcex94nxc2x7d of the phase plate is set to be 10 to 200 nm. A liquid crystal drive voltage is regulated to be equal to or smaller than a voltage at which a light transmittance with respect to one obtained at a no voltage applied state when no phase plate is used is set to be 1%.
However, in these prior arts, although a viewing angle with respect to contrast is improved, an image becomes disadvantageously yellowish when viewed from an oblique direction. Improvement on gradation inversion from a downward direction is insufficient.
It is an object of the present invention to provide a liquid crystal display device which can prevent a change in color to yellow when viewed from an oblique direction, and prevent gradation degradation from a downward direction.
According to one aspect of the present invention a liquid crystal display device may have a light source, a liquid crystal panel arranged on the light source, and a phase compensation film arranged on the liquid crystal panel. The liquid crystal panel may include a twisted nematic liquid crystal layer having a twisted angle of substantially 90xc2x0 and a helical pitch equal to or more than 60 xcexcm.
According to one aspect of the present invention, since the helical pitch of the twisted nematic liquid crystal layer is properly regulated, coloring of yellow made when viewed from an oblique direction can be suppressed. In addition, gradation inversion from a downward direction can be suppressed.